8.3: Thermochemistry of Substitution Reactions
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- 22212
Ionic or polar reactions of alkyl halides rarely are observed in the vapor phase because the energy required to dissociate a carbon-halogen bond heterolytically is almost prohibitively high. For example, while the heat of dissociation of chloromethane to a methyl radical and a chlorine atom is \(84 \: \text{kcal mol}^{-1}\) (Table 4-6), dissociation to a methyl cation and a chloride ion requires about \(227 \: \text{kcal mol}^{-1}\):
However, the heat of ionic dissociation of methyl chloride in aqueous solution is estimated to be \(63 \: \text{kcal}\), and while this reaction is still substantially endothermic, it requires about \(227 - 63 = 164 \: \text{kcal}\) less energy than in the gas phase:
The reason is that ions are much more stable in water than in the gas phase; for example, the transfer of a chloride ion from the gas to water is exothermic by \(-85 \: \text{kcal}\). The \(\Delta H^\text{0}\) value for the corresponding transfer of a methyl cation, \(CH_3^\oplus\), is not known with certainty, but is about \(-80 \: \text{kcal}\). These ionic solvation energies are clearly large. In contrast, the \(\Delta H^\text{0}\) for solution of methyl chloride in water is small (about \(1 \: \text{kcal}\)). We can use these data to calculate the heat of ionic dissociation of chloromethane in water:
Thermochemical data for the solvation of ions as used in the preceding calculations are difficult to measure and even to estimate. Therefore this kind of calculation of \(\Delta H^\text{0}\) for ionic reactions involving organic molecules in solution usually cannot be made. As a result, we have considerably fewer possibilities to assess the thermodynamic feasibility of the individual steps of polar reactions in solution than we do of vapor-phase radical processes. Bond energies are not of much use in predicting or explaining reactivity in ionic reactions unless we have some information that can be used to translate gas-phase \(\Delta H^\text{0}\) values to solution \(\Delta H^\text{0}\) values.
\(^2\)Calculated from the following data:
Contributors and Attributions
John D. Robert and Marjorie C. Caserio (1977) Basic Principles of Organic Chemistry, second edition. W. A. Benjamin, Inc. , Menlo Park, CA. ISBN 0-8053-8329-8. This content is copyrighted under the following conditions, "You are granted permission for individual, educational, research and non-commercial reproduction, distribution, display and performance of this work in any format."